feat(engine): default checkpointer/session to in-memory when unset (LangGraph parity with ADK)

[Freezaa9]

Summary

The LangGraph adapter leaves self._checkpointer = None when the YAML omits the checkpointer: block, which silently disables conversation history, session listing, thread state, and the AG-UI threadId/history capability flags. ADK already defaults to InMemorySessionService / InMemoryMemoryService in the same situation. The engine should provide the same in-memory default for LangGraph so a minimal config "just works" — matching how a developer would expect a freshly-installed engine to behave.

Grounding in code

LangGraph — no default (current behavior)

libs/idun_agent_engine/src/idun_agent_engine/agent/langgraph/langgraph.py:419-453

async def _setup_persistence(self) -> None:
    if not self._configuration:
        return

    if self._configuration.checkpointer:  # ← if None, nothing happens
        if isinstance(self._configuration.checkpointer, SqliteCheckpointConfig):
            ...
        elif isinstance(self._configuration.checkpointer, InMemoryCheckpointConfig):
            self._checkpointer = InMemorySaver()
            ...
        elif isinstance(self._configuration.checkpointer, PostgresCheckpointConfig):
            ...

Downstream effects when self._checkpointer is None:

• langgraph.py:866-874 — has_checkpointer = self._checkpointer is not None drives capabilities.history and capabilities.threadId. Both flip to False.
• langgraph.py:1010 — list_sessions() returns [] immediately.
• langgraph.py:1047 — get_session() short-circuits on not self._checkpointer.
• LangGraph itself cannot carry state between turns at all without a checkpointer; multi-turn chats lose context silently.

Schema: libs/idun_agent_schema/src/idun_agent_schema/engine/langgraph.py:76

checkpointer: CheckpointConfig | None = None

So a minimal config like:

agent:
  type: LANGGRAPH
  config:
    name: My Agent
    graph_definition: ./agent.py:graph

is valid and lands the user in the no-checkpointer state with no warning.

ADK — already defaults to in-memory

libs/idun_agent_engine/src/idun_agent_engine/agent/adk/adk.py:389-431

async def _initialize_session_service(self) -> None:
    ...
    if not self._configuration.session_service:
        # Default to InMemory if not specified
        self._session_service = InMemorySessionService()
        return
    ...

async def _initialize_memory_service(self) -> None:
    ...
    if not self._configuration.memory_service:
        # Default to InMemory if not specified
        self._memory_service = InMemoryMemoryService()
        return

Proposed change

In _setup_persistence() (LangGraph adapter), when self._configuration.checkpointer is None, default to InMemorySaver() and record an InMemoryCheckpointConfig-equivalent entry in self._infos[\"checkpointer\"]. Log an INFO line so operators know in-memory was chosen as a fallback and that state is process-local (not durable across restarts).

Sketch:

if self._configuration.checkpointer:
    # existing branches unchanged
    ...
else:
    self._checkpointer = InMemorySaver()
    self._infos[\"checkpointer\"] = InMemoryCheckpointConfig(type=\"memory\").model_dump()
    logger.info(
        \"No checkpointer configured for LangGraph agent; defaulting to in-memory \"
        \"(state is not persisted across process restarts).\"
    )

Acceptance criteria

• LangGraph adapter wires an InMemorySaver when agent.config.checkpointer is unset, with an INFO log indicating the fallback.
• GET /agent/capabilities returns history: true and threadId: true for a minimal LangGraph config with no checkpointer: block.
• GET /agent/sessions returns sessions after a multi-turn conversation under the same config.
• Multi-turn conversations preserve LangGraph state across /agent/run invocations on the same thread.
• Unit test in libs/idun_agent_engine/tests/ covers the unset-checkpointer path, asserting both _checkpointer is not None and capabilities.history is True.
• Docs updated:
  • libs/idun_agent_engine/CLAUDE.md (Configuration Flow / LangGraph Key Details) — note the default.
  • docs/ (Mintlify config reference) — mention the default and that it is non-durable.

Notes / tradeoffs

• This is a behavioral change for users currently relying on "no checkpointer = no history". Risk is low because the prior behavior was almost always a foot-gun (silent loss of conversation state); however, the INFO log makes the choice visible.
• Keeps parity with ADK, which already does the same.
• Does not change defaults for durable backends — users still opt into SQLite/Postgres explicitly.